In a UMTS (Universal Mobile Telecommunications System) network, for the purposes of achieving higher-speed data rates, providing low delay and so on, long-term evolution (LTE) has been standardized (see Non Patent Literature 1). In LTE, as multi access schemes, an OFDMA (Orthogonal Frequency Division Multiple Access)-based scheme is used for downlink and an SC-FDMA (Single Carrier Frequency Division Multiple Access)-based scheme is used for uplink. For the purposes of achieving further broadbandization and higher speed, successor systems to LTE have been also studied and standardized (Rel. 10/11) (for example, such a system is also called “LTE advanced” or “LTE enhancement” (hereinafter referred to as “LTE-A”)).
In the LTE-A system, HetNet (Heterogeneous Network) has been also studied in which a macro cell is formed having a wide coverage area of about several kilo meter radius, and a small cell (for example, pico cell, femto cell or the like) having a local coverage area of about several ten meter radius is formed within the macro cell. In the HetNet environment, it has been considered that the macro cell (macro base station) and the small cell (small base station) use not only carriers of the same frequency band but also carriers of different frequency bands.
Further, in a future radio communication system (Rel. 12 or later), the LTE system has been considered as operating not only in a licensed frequency band but also in an unlicensed frequency band (license-not-required frequency band) (which system is called LTE-U: LTE Unlicensed). When the LTE-U is operated on the basis of cooperation of the licensed band LTE (Licensed LTE), it is called LAA (Licensed-Assisted Access) or LAA-LTE. A system operating LTE/LTE-A in the unlicensed band is sometimes called “LAA” collectively.
The licensed band is a band that is permitted to be used exclusively by a specific operator (carrier), while the unlicensed band (also called “non-licensed band”) is a band in which a radio base station is able to be installed without restriction to a specific operator. As the unlicensed band, a 2.4 GHz or 5 GHz band, in which Wi-Fi (registered trademark) and Bluetooth (registered trademark) is usable, and a 60 GHz band, in which milli-meter wave is usable, have been considered to be used. This unlicensed band has been also considered to be applied to a small cell.